section - Bitsavers

More documents

Recommendations

Info

PHASE LOCKED LOOP APPLICATIONSTRANSLATION LOOP FOR PRECISE FM (5618,5628)A translation loop mixes the output of two oscillators andproduces a signal whose frequency is equal to the sum ordifference of the two. In the most useful application of thiscircuit, one oscillator is a precise crystal-controlledoscillator and the second is a low frequency voltagecontrolledoscillator so that the loop output is a FM signalwhose center frequency is slightly offset from the crystaloscillator frequency. Since the offset oscillator suppliesonly a small percentage of the final output frequency, itneed not be as precise as the crystal oscillator.Such a loop is shown in Figure 8-50a. The VCO is drivenuntil the filtered low frequency component of the P02output is equal to the offset frequency f m . When thisoccurs, lockup is achieved and the VCO output is eitherfR + fm or fR - f m. By adjusting the VCO free-runningslightly above f m , the latter case can be eliminated. If fmis frequency modulated, then the output will also befrequency modulated since it has the same absolutedeviation.TRANSLATION LOOP FOR PRECISION TV INTERMEDIATE-FREQUENCY FM GENERATORVCOFigure 8-50aFigure 8-50b shows a translation loop made from a 561Band 562B. It is designed to produce a 4.5MHz signal with adeviation of ±25kHz. The 561 B serves as the VCO andPO 1; the 562B serves as the crystal oscillator and PD2. A4.400MHz crystal controls the reference frequency fRoThe offset frequency fm is 100kHz frequency modulated±25kHz at a modulation frequency of 400Hz. Theaccuracy of the output frequency is that of the referenceoscillator plus that of the offset oscillator; since fm is asmall percentage (2%) of fR, its stability can be considerablyless than that of the crystal oscillator. In this case, fmcan be provided by a 566 VCO modulated, if desired, by asecond 566. (The triangle wave 566 output results in aconstant df/dt.)TRANSLATION LOOP FOR PRECISION TV INTERMEDIATE - FREQUENCY FM GENERATOR+18fm = 100KHz ± lIf(150MV-RMS)+18fR4.400 MHz cCRYSTAL-=5pF680fm11 5628.05330 14-=.05P LPFNO.243pF-=SET VCO1-15pFfR + fm =4.5MHz ± 6fFigure 8-50b55
PHASE LOCKED LOOP APPLICATIONSSpecial layout precautions are required to be sure that nohigh frequency coupling occurs via grounds or powersupply lines. The circuit is adjusted by trimming the 562VCO trimmer capacitor until the beat note present at testpoint 1 has the same frequency as f m throughout thedeviation range (f m can be deviated by hand or very slowly,say, at a 1 Hz rate, to observe that the beat note does notbreak up during sweep. If the beat note is lost at eitherextreme, adjust the VCO trimmer. If the full deviationcannot be obtained, decrease the 562 low pass filtercapacitor sl ightly. Connect a cou nter to the output to besure the loop is locked to fR + fm and not fR - fm (unlessthe latter is desired).Naturally, the component values given may be altered forother applications. Note that as fm is made a smaller andsmaller percentage of the total output frequency, itbecomes difficult to prevent locking in the fR - fm modesince the 562 lock range will likely include both fR - fmand fR + f m . However, if fm is made too large a portion ofthe output frequency, then overall stability suffers unlessf m is also qu ite precise.PHASE LOCKED FSK DEMODULATORS (5608,565)FSK refers to data transmission by means of a carrierwhich is shifted between two preset frequencies. Thisfrequency shift is usually accomplished by driving a VCOwith the binary data signal so that the two resultingfrequencies correspond to the "0" and "1" states(commonly called space and mark) of the binary datasignal.The 560B phase locked loop can be used as a recelvmgconverter to demodulate FSK audio tones and to provide ashifting dc voltage to initiate mark or space code elements.The PLL can replace the bulky audio filters andundependable relay circuits previously used for thisapplication. Connection of the 560B PLL as a FSKdemodulator is illustrated in Figure 8-51.The system functions by locking-on and tracking theoutput frequency of the receiver. The demodulatorfrequency shift appears at pin 9 as a direct-current voltageof about 60mV amplitude and must be amplified andsignal-conditioned to interface with the printer. The inputvoltage at pin 12 should be from 30mV to 2V peak-to-peak,square or sine wave. Pin 10, the de-emphasis terminal, isused for bandshaping. The capacitor connected betweenthis terminal and ground bypasses unwanted highfrequency noise to ground. Pin 9 is the output (approximately60mV dc) which is amplified, conditioned and fedto a voltage comparator amplifier (N5710) to provide theproper voltages for interfacing with the printer. Thisspecific circuit was designed to match the Bell 103C and103D Data Phones. When modifying this circuit toaccommodate other systems, maintain the resistance toground from pin 9 at approximately 15Q. Pins 3 and 2 arethe connections for the external capacitor that determinethe free-running frequency of the VCO. The 0.33MF· valueindicated provides a VCO frequency, f o ' of approximately1060Hz. The value of the timing capacitor can beCo = 300pFcalculated by use of the following equation:where fo is in Hertz.The output has a swing of 2V peak-to-peak, over a 0 to 600baud input FSK rate, with less than 10% jitter at thecomparator output. The circuit is operative over atemperature range of 0° to 75 0 C with a total drift ofapproximately 1 OOmV over the temperature range.A simple scheme using the 565 to receive FSK signals of1070Hz and 1270Hz is shown in Figure 8-52. As the signalappears at the input, the loop locks to the input frequencyand tracks it between the two frequencies with acorresponding dc shift at the output.FSK DECODER USING THE 565FSK DEMODULATOR USING 560810K10KL......;.-+---------+----o-5VFigure 8-52OUTPUTr----"."IIr-+--+---+--......... --o-6VFigure 8-51The loop filter capacitor C2 is chosen to set the properovershoot on the output and a three-stage RC ladder filteris used to remove the sum frequency component. Theband edge of the ladder filter is chosen to be approximatelyhalf-way between the maximum keying rate (30056
Page 2 and 3:
TABLE OF CONTENTSSECTION TITLE PAGE
Page 4:
SECTIONINTRODUCTIONPHASE LOCKED LOO
Page 7 and 8: USER'S QUICK-LOOK GUIDE TO SIGNETIC
Page 10: SECTIONTHE PHASE LOCKED LOOP PRINCI
Page 13 and 14: PHASE LOCKED LOOP APPLICATIONSIt is
Page 15 and 16: PHASE LOCKED LOOP APPLICATIONSROOT
Page 17 and 18: PHASE LOCKED LOOP APPLICATIONSINTEG
Page 19 and 20: ~PHASE LOCKED LOOP APPLICATIONSFUNC
Page 21 and 22: PHASE LOCKED LOOP APPLICATIONSFREQU
Page 24 and 25: PHASE LOCKED LOOP APPLICATIONSGENER
Page 26 and 27: PHASE LOCKED LOOP APPLICATIONSPROBA
Page 28 and 29: ~PHASE LOCKED LOOP APPLICATIONSCapt
Page 30 and 31: PHASE LOCKED LOOP APPLICATIONSTRANS
Page 32 and 33: PHASE LOCKED LOOP APPLICATIONSThe V
Page 34 and 35: PHASE LOCKED LOOP APPLICATIONSINTER
Page 36 and 37: PHASE LOCKED LOOP APPLICATIONSIt is
Page 38 and 39: PHASE LOCKED LOOP APPLICATIONSWhen
Page 40 and 41: PHASE LOCKED LOOP APPLICATIONSSIMPL
Page 42 and 43: PHASE LOCKED LOOP APPLICATIONSSCHEM
Page 44 and 45: PHASE LOCKED LOOP APPLICATIONSNARRO
Page 46 and 47: PHASE LOCKED LOOP APPLICATIONSTwo e
Page 48: PHASE LOCKED LOOP APPLICATIONSd.)e.
Page 52 and 53: PHASE LOCKED LOOP APPLICATIONSFM IF
Page 54 and 55: PHASE LOCKED LOOP APPLICATIONSPHASE
Page 58 and 59: PHASE LOCKED LOOP APPLICATIONSbaud
Page 60 and 61: PHASE LOCKED LOOP APPLICATIONSFigur
Page 62 and 63: PHASE LOCKED LOOP APPLICATIONSinput
Page 64 and 65: PHASE LOCKED LOOP APPLICATIONfTRIAN
Page 66 and 67: PHASE LOCKED LOOP APPLICATIONSRF-FM
Page 68 and 69: PHASE LOCKED LOOP APPLICATIONSVHF O
Page 70 and 71: PHASE LOCKED LOOP APPLICATIONSPRECI
Page 72 and 73: PHASE LOCKED LOOP APPLICATIONSFSK D
Page 74 and 75: PHASE LOCKED LOOP APPLICATIONSThe c
Page 76 and 77: PHASE LOCKED LOOP APPLICATIONSFLUTT
Page 78 and 79: PHASE LOCKED LOOP APPLICATIONSSALES
Page 80 and 81: PHASE LOCKED LOOP APPLICATIONSINTER
show all

section - Bitsavers

Create successful ePaper yourself

Delete template?

Save as template?